A owns 10 hectares of land on which a house and barn have been built, and on which there is a crop of wheat growing.

More than one 'thing' is mentioned in this situation. Firstly, there is the area of land, secondly, the buildings, and thirdly, the growing crop of wheat. Although these are three very different things, they have something in common, namely, that they are considered to be 'fixed' rather than 'moveable'. There is no way that any of these things could actually be moved to a different location - the law thus treats such things as 'immoveable' or 'fixed property'.

Because land is the prime example of immoveable property, this whole category of property is sometimes simply referred to as 'land'. But you must remember that it includes all things that are fixed and cannot be moved to a different location without harming or destroying them - things built on land (houses, walls, outbuildings etc) and things growing on land (trees, crops etc).